Method of making friction elements and bond therefor



Nov. 9, 1948; v 'w. A. BLUME ETAL 2,453,188

METHODKOF MAKING FRIQTION ELEMENTS AND `BOND THEREFOR Filed June-28, 1943 v l 5 Sheets-Sheet 1 QS .1 Ng 8 M L nl, Q@ N` Q) W jk i m NK L' has N fas i VS u N s EN Q@ J 5 q: E s w l N J 1%" QR METHOD OF MAKING FRICTIN ELEMENTS AND BOND THEREFOR Filed June 28, A1945 I w. A. BLUME ErAL Nov. 9, 1948.

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METHOD OFl MAKING FRICTION ELEMENTS AND BOND THEREFOR y 2102,29294! f ,zzzaqg/fao] Patented Nov. 9, 1948 METHOD OF MAKING FRICTION ELEMENTS AND BOND THEREFOR William A. Blume, Farmington, and tay Spokes, Ann Arbor, Mich., assignors to American Brake Shoe Company, New York, N. Y., a

corporation of Delaware Application June 28, 1943, Serial No. 492,562

3 Claims. (Cl. 10G-3.6)

This invention relates to friction elements such as are employed in the brakes and clutches of automotive vehicles, aircraft, machinery and the like and to the art of producing the same.

Friction elements of the character to which this invention pertains generally include friction material, such as asbestos, either in the form of woven tape or fabric, or in a somewhat loose or socalled felted form, and a bonding agent, which may be comprised of one or more vegetable drying oilsror` the like, and often other ingredients for controlling or imparting particular friction or other characteristics to the bond or to the resulting friction element.

Among the vegetable drying oils Which have been employed heretofore as bonding agents in friction elements are tung or China-Wood oil, linseed oil, perilla oil, and the like and also China- Wood oil modified phenol-formaldehyde resins. Such oils are commonly prenbodied, prior to use in' or as bonding agents in friction elements, to impart desired body or viscosity thereto and the final bodying and hardening of the bonding oil is effected, at least in part, by heat after the friction material has been impregnated with the bonding oil. Such final bodying and hardening of the bond may be effected by heat polymerization and oxidation, in situ, or by heat and dulphurization, or by a combination of such methods, and it is with friction elements employing bonding agents which are cured primarily by sulphurization that the present invention is concerned.

The bodying of such vegetable drying oils has been effected in various Ways as, for example, pre-b-odying by partial hydrogenation of tung or China-wood oil, followed by nal bodying by heat polymerization and oxidation, in situ, that is, after the friction material has been impregnated with ,the bonding oil, as in Seigle Patent No. 1,395,- 744, or pre-bodying of linseed oil by heat-polymerization to form a so-called purely (heat) polymerized oil, followed by final bodying by heat and sulphurization after the friction material has been impregnated with the purely polymerized linseed oil, with or Without other ingredients. The present invention is primarily concerned with friction elements of the character in which the final bodying and hardening of the bonding oil is effected by heat and sulphurization, regardless of the method by which the pre-bodying of the bonding oil is effected because certain diiiiculties have been encountered heretofore in connection with the sulphurization of such partially bodied vegetable drying oils used in or as bonding agents in friction elements.

The customary method of incorporating sulphur into such pre-bodied vegetable drying oils liasfibeen -by mechanically mixing thesulphur with the pre-bodied bonding oil prior to incor- A' nora-ting the thus formed sulphur-oil mixture or fabric acts as a lter and filters the sulphur with the friction material. However, this method has not been satisfactory for a number of reasons and' among which are: (l) It is impossible to .uniformly impregnate woven asbestos, that is,

so-called Woven asbestos tapeor fabric, Which is commonly employed as friction material in making friction elements, with a mechanical mixture of sulphur in partially bodied vegetable drying oil for the reason that the woven asbestos tape out of such sulphur-oil mixtures; (2) It is difficult to obtain. a uniform dispersion of sulphur in a partially bodied vegetable drying oil by any known method of mechanically mixing the sul phur and oil with the result that it is impossible by such methods to obtain a uniform distribution oi' the sulphur throughout friction material impregnated with such a solvent thinned ysulphur-` eil mixture; and 3) 'It is impossible to subject woven asbestos tape or fabric, which usually con'- tains about twenty percent (20%) by Weight of cotton, to a temperature, and for a period of time, sufficient to heat polymerize a vegetable drying oil such, for example, as China-wood oil, with which such woven asbestos might be impregnated, since the temperature and heating time required to heat polymerize the oil would have a deleterious effect upon the asbestos-cotton Woven fabric. Hence in the practice of the present invention the vegetable drying oil is heat polymerized, as far as is practicable, prior to the time the sulphur is dissolved therein and the sulphur-l oil mixture is employed as a bonding agent in friction elements.

A further difiiculty incidental to prior art attempts, to impregnate Woven asbestos tape or,

fabric with a mechanically mixed dispersion lof sulphur in pre-bodied vegetable drying oil is that since it is impossible to impregnate such material with a mechanical bonding mixture of vegetable drying oil and sulphur only the external surface portions of the woven asbestos fabric are impregnated with such a bonding mixture and the internal mass or body remains unimpregnated i so that such methods have not been successful.

Hence it will be seen that the bonding of friction elements employing Woven asbestos or socalled asbestos tape or fabric as friction mate' rial has been limited to the use of those bodied vegetable drying oils, such as China-wood or tung oil, which cure primarily by polymerization, and

other vegetable drying oils which cure primarily 4 by ox-idation, and to the use of other bonding agents such as asphaltic pitches and the like, which do not require sulphurizati-on to effect the final curing or hardening thereof and the final hardening or curing of which is effected largely by heat polymerization and/or oxidation, in situ.

It has not, therefore, been possible heretofore to4 canales.

friction elements are obtained by heating an intimate mixture of finely divided sulphur in and with solvent-thinned bodied vegetable drying oil, of suitable body or viscosity, to a temperature sufficient to dissolve the finely divided sulphur physically in the oil but not sufficiently high to cause the sulphur to combine chemically with the oil 4which would cause excessive bodying of the oil and would render ity impossible to impregnate woven asbestos fabric with such a heavily bodied oil.

We have also found that when friction material, such as Woven or felted asbestos, is impregnated with the thus formed vegetable drying oilsulphur solution substantially uniform distribution or impregnation and cure of the sulphurized bonding oil throughout the entire mass or thickness of the friction material is obtained during vthe final heat curing operation, thereby overcoming a difficulty commonly experienced heretofore in the use of vegetable drying oils, such as tung or China-wood oil, which have been used heretofore to impregnate woven asbestos tape or fabric and which partially or completely oxidize during the heat-curing operation.

In the practice of the present invention, and in making the new friction elements, we may employ any partially bodied and sulphurizable vegetable drying oil, or a mixture of such oils, which has been bodied sufficiently or has a suitable viscosity to enable it to be employed in or as a bonding agent in friction elements. Thus, a typical example of such an oil is the yaforesaid so-called purely (heat) polymeri'zed` linseed oil, of suitable viscosity, either alone or modified with other materials, such, for example, as an oil miscible resin of the phenolicaldehyde type, an example of which is an oil modified Bakelite type resin, with or without other friction-controlling ingredients such, for example, asy graphite, pitch or the like.

Thus we may, for example, employ the aforesaid so-called purely (heat) polymerized linseed oil, which is also known as purely polymerized but not otherwise changed linseed oil, of lsuitable viscosity, suchan oil being one which has been subjected, concurrently with the vheat-polymerizing operation, to a secondary or ancillary process for removing substantially all of the free fatty acid content'as formed in the oil during and as a result of the heat-polymerizing operation as by having carbon dioxide or other inert gas passed through the oil during the heat-polymerizng operation.

If desired, however, we may also employ in the practice of the present invention a heat-polymerized vegetable drying oil such, for example, as linseed oil, of suitable viscosity, in which substantially all of the free fatty acid content formed in the oil during and as a result of the heatpolymerizing operation is retained therein rather than being removed therefrom, or we may employ a mixture of the aforesaid different types of heatpolymerized vegetable drying oils of suitable viscosity.

Another feature of the present invention is that by controlling the amount of sulphur employed in 'making the new vegetable drying oil-sulphur bonding solution we are 'enabled to use in preparing said bonding solution la heavily bodied sulphurizable vegetable drying oil which has been bodied by heat polymerization until it is highly viscous and tacky but is 4substantially free from gel formation and is completely soluble in petroleum thinner. The use of such a heavily bodied oil is particularly advantageous'in making i'ric-l tion .elements from woven asbestos fabric containing a substantial proportion of cotton since such oils, being initially highly bodied, require a relatively low temperature and a relatively short curing time to effect the final heat curing or bodying and hardening of the sulphurized bonding oil, thereby minimizing the deleterious effect of the heat-curing operation upon the cotton content, especially of the asbestos fabric, prior to nal cure of the impregnant and resulting protection aorded the fibers by being so coated.

In the practice of the present invention, we preferably intimately mix nely divided sulphur and the selected bodied sulphurizable vegetable drying oil at atmospheric temperature and we then heat the mixture thus prepared to a temperature suiciently high to melt the finely divided sulphur, and thus dissolve the sulphur physically in the oil, care being taken to avoid temperatures sufficiently high to cause the sulphur to combine chemically with the oil which would cause excessive bodying of the selected bonding oil and render it impossible to impregnate woven asbestos fabric or loosely felted asbestos therewith. Thus we have found that a temperature which is satisfactory for this purpose is a temperature of not substantially less than C. and not substantially higher than C., since within this rather critical range of temperatures the sulphur dissolves physically in the heat polymerized and bodied vegetable drying oil without combiningchemically therewith.

Friction material such, for example, nas woven or felted asbestos, with or without other frictioncontrolling ingredients or materials, is then thoroughly impregnated, in any suitable manner, with the thus formed vegetable drying oil-sulphur solution. In this manner there is obtained substantially uniform impregnation and distribution of the bonding oil-sulphur solution throughout the entire mass or thickness of the woven asbestos fabric, or loosely felted asbestos, or other fibrous, porous or like friction material, which may be used in making the new friction elements.

Another advantage of such uniform impregnation of the presulphurized bonding oil throughout the mass of asbestos is that after the final curing operation the sulphurized and hardened bonding oil not only binds the asbestos fibers together but affords a new and improved friction element having uniform frictional and wear characteristics throughout its usefull life due, to a large extent, to the uniformity of the impregnant or binder both in distribution and composition,

Another advantage of the new meth-od of making friction elements is that it enables the sulphurization of the selected sulphurizable vegetable drying oil employed as the lbonding agent to be readily ycontrolled since sufficient 'sulphur may be incorporated int-o the sulphur-oil Ibonding solution to .assure complete sulphuriza-tion of the selecte'd sulphurizable vegetable drying oil.

In this manner new and improved friction elements are VVobtained having the desirable advantages and characteristics hereinbefore referred A typical example of a suitable formula and` method which may be followed in making friction elements in lthe practice of the :present invention, and employing woven asbestos tape or fabric as the loose friction material, is the following;

Example 433 pounds of the new sulphur-oil bonding soi lution'were preparedby intimately mixing l72,2,

the aforesaid purely heat-polymerized liriseed oil.

from whichlsubstantia'l-ly all of the free fatty acid content Vformed in the oil during. and a result ofthe heat-polymerizing operation Wasremoved. `therefrom `as formed as by. havinganxinert gas suchas Icarbon dioxide. passed. through theoil during .the .heat-polymerizing operation.. The selected oil had an initial viscosity of approximately`.800 poises, at atmospheric temperature, :and was highly viscous 4and tacky -but was' sub.- stan-tially free of gel. `formation and was. com- .pletely soluble in petroleum thinner.v However, in the .practice of the present inventi-on the selected bonding. oil is not dissolved in petroleum thinner or. other solvent prior Ato or after mixturewith the sulphur and reference herein to the solubility ofthe selected bonding oil in petroleum thinner 4is .merely by way of describing one. of the initial physical characteristics of 'such oils.

'The sulphur-oil mixture thus prepared was thereupon heated at a. substantially constant temperature of 135 C., while stirring, until all of the sulphur dissolved in the oil. The result was a relatively thin and free-flowing solution of sulphur'in the bonding oil and-which readily penetrates woven asbestos fabric or tape or likewoven friction material.

The resulting sulphur-oil, solution contained 16.7.percent of sulphur and 83.3 percent of oil of the total of sulphur and oil, by weight.

Awstrip vof woven asbestos tape 31A Wide and 3/8 thick and of a preselected length was heated to; atempera'ture of 135 C. and was then immersed for a suitalble length of time, namely, about six minutes, in the thus prepared and heated sulphur-oil solution While maintaining the sulphur-oil solution at a substantially constant temperature of 135 C., while stirring, and pref-v erably pressingr the woven asbestos fabric-once -ortWi-ce, during the immersion operation, to remove excess of the sulphur-oil bonding solution. The thus impregnated strip of woven asbestos tape was then removed from the sulphur-oil solution and then lfreed lfrom excess oil and sufbjected to a pressure of from about 0.7 ton per' square inch in orderto removeA entrapped or occluded air, and excess impregnant aswell as` for theV purpose of controlling the thickness of the resulting friction elements. impregnated but uncured specimen showed a Weightincrease ofv 16.1 percent due to penetration of the `sulphur-oil bonding solution into the Woven asbestos fabric.

The specimen of impregnated woven asbestos tape was then subjected to gradually increasing temperatures to effect the final heat curing or hardening and bodying of the sulphurized bonding oil, namely, three hours at each of the fol- 'lowing temperatures: 150 F., 175 F., 200 F., 225 F., 250 F., 275 F., 300 F., and 310 F., it being understood, of course, that the temperature and time required to effect theV final (body- Iing and hardening ror heat ycuring of the thus sulphurized bonding oi'l will Vary with the composition of` the bond, the -particular `sulphurizalble vegetable drying oil selected, and the initial viscosity thereof, and other variable factors.

The heating of the friction material before it' is introduced'int-o the sulphur-oil solution prevents sulphur from passing out of solution with the oil which would occur -ifthe friction material were not thus pre-heated prior to its introduction into the oil, and it also removes entrappedi or occludedv air and moisture from the friction. ma-

TheY resulting teri-al. and. thus assists.` in: securing: moreffurrz-w formimpregnation.. of: the. sulphurS-oil'` solutionI throughout the: mass or bodyzof.. frictionmateriali2 The. vegetable -drying-..oil`-su1phur 'bondingfsolur-l tion referred to in thek foregoingillustrative "ex-f ample maybe: modified., if desired, witlriian;v oil.. miscible. phenolicfaldehydetype resinsuch', fon` example,. as an oil modified: Bakelite. type. resina It `willfbe understood that while in:.thefore..f going. typical. and! illustrative: exampleA weahav'e" indicated the use of a preliminary pressure treat"- 'mentl ofthe? impregnated 'friction material; toirem'ove occluded or entrappeda air andrexcess'of the sulphur-bonding oil. impregnant, as: well as: for; thepurpose of controlling the thickness.v of there:- sulting friction elements, this' pre'limina'ry"press. Y sure vtreatment isv not indispensablerinfor tothepractice of 'thepresent invention, andi may, ifa desired, ibe eliminated entirely within the1.pur-' view of the present invention.

Inorder to.' illustrate `the improved: properties and characteristics of friction. elements: made .a'c-l cording to thev practice of thev presentv inventionf friction tests-were run upon specimensof friction2 elements made according tothe practice." of the present invention. and upon comparable-especimens of priorart friction elements'.l 4

Figs. 1' to 4, inclusive', of thefdrawing: are. graphs,1.some of. which illustrate thefresults: of.; tests made upon specimens or friction: elements made according to the. practice of thepresent invention, and others of which.illustratethefresults of tests made.- upon comparable friction :ele-Av ments made according' toipriorr arti practices;

In the graphs shown'. in the drawings, theiabscissaerepresentv the results of friction. tests?, statedin: number of braking applications-each'. oftwo minutes duration, andzatl various applica tion speeds, expressed'intermsoffeetper minute; and the ordinates represent theresult's oft'estsa expressedinf terms of thecoei'cients offrction of the frictionelementstested.'v

The tests of which. the: results aregraphicallyl shown in Figs; 1 to; inclusive,A wereconducted;

on' al special testing'machine' wlrererrspe(simensv of? friction elements compounded?asl aforesaid?vv-ere successively secured. in at holder' and forced: by:` controlled?. variable hydraulic pressure.y againstf; they surface' ofv a' rotating drum embodiedfin the; machine; the speed of the-rotating drum being' varied' to simulate speeds of.automotive?vehicles?l upon. the brakes of which suchl frictionzfelernentsv are used. The friction developed by the specie-v mens is'rneasured4` as forceby= hydraulically-operated: measure apparatusand the* coefcien'ts. of i friction are calculated' therefrom; v

It :should: benoted', in: this regard; thatlin` con-V ducting tests, suchras those to`which the-graphs?- pertain, on at special testing machine; there may: be variationsin rea-dings taken at'diierent .times-V under identical. test conditions;l attributable,` as is. well understood; to the inherent characteristics ofi 'the entailed' mechanical devices 'and other related circumstances. Thus, in consideringrthef accompanying-graphs, andi the following discussion thereof, it should be rememberedithatfeach-l of these graphs illustrates the averageof repeatedtestsv and vshows a trend4 toward a` particu-f lar result. rather than specific.' conditions Wl'iichf;

` maybe"exactly-duplicated time-after time.'

lsubstantially the same viscosity under similar vsubstantially free of gel formation while being completely soluble in a suitable volatile solvent, such as petroleum thinner.

Thus, Fig. 1 is a group of three graphs which show the results of friction tests made upon a specimen of a friction element made according to prior art practice, and employing woven asbestos tape of the same character, width and thickness, as that employed in making those specimens of friction elements which were made according to the present invention, and to which reference will be made hereinafter.

In preparing the specimen, the results of which are shown in Fig. 1, raw tung or China-wood oil was employed as the bonding oil, and the woven asbestos tape or fabric impregnated with this bonding v'oil showed a weight increase of 17.5% after impregnation due to impregnation by the bonding oil. The specimen was subjected to a preliminary curing operation at a pressure of 0.7 ton per square inch and at a temperature of 250 F. for a period of twenty minutes, the final curing operation being conducted in an open oven so as to .effect the final curing or bodying and hardening of the unsulphurized oil largely by oxidation according to prior art practice.

It will be noted, by reference to the graphs shown in Fig. 1. that the specimen of a friction element tested stood up fairly well under the first test at a braking or application speed of 1000 feet per minute. as indicated by graph I in Fig. l, feet per minutes, as indicated by graph 1 in Fig. l, and likewise withstood fairly well the second test at a braking or application speed of 2000 feet per minute, as indicated by graph 2 in Fig. 1. This is shown by the relatively uniform coefficient of frictionthroughout thetests,as shown in graphs 1 and 2 in Fig. 1. It will be noted, however, that this specimen disintegrated entirely during the third and more severe test involving a braking or application speed of 3000 feet per minute, as indicated by graph 3 in Fig. 1, and as shown by the practically vertical rise in the coefficient of friction in the friction element tested. This indicates a substantially complete break-down of the friction element resulting, in part, from non-uniform impregnation and cure of the bonding mixture throughout the friction material.

Fig. 2 is a group of three graphs which show the results of friction tests made upon a specimen of a friction element employing as friction material Woven asbestos tape of the same character and thickness as that employed. in making the specimen of a friction element the results of tests upon which are shown in Fig. 1. In this instance, however, the bonding agent employed was raw or unpolymerized linseed oil having finely divided sulphur mixed mechanically therewith at atmospheric temperatures to provide a mechanical mixture or dispersion of sulphur in the oil. The preliminary and final curing of the bonding oil was carried out under the same conditions of temperature, etc., as hereinbefore referred to in connection with the description of the graphs shown in Fig. 1 and the specimen of a friction element therein referred to. The-'percentage of sulphur in the mechanical sulphur-oil mixture was 16.7 percent of the total weight of the sulphuroil mixtureand the increase in weight f the fric tion material after impregnation was 17.4 percent due primarily to the impregnation of the oil itself 10 rather than of the sulphur mechanically mixed therein.

It will be noted from Fig. 2 that the friction element tested withstood fairly well the iirst test at an application or braking speed of 1000 feet per minute, as indicated in graph 4, and also withstood quite Well the second test at an appli cation speed of 2000 feet per minute, as shown by graph 5 in Fig. 2. It will be noted. however, that, like the friction element referred to in Fig. 1, the specimen referred to in Fig. 2 disintegrated completely at the more severe test involving an application speed of 3000 feet per minute, as shown by graph 6 in Fig. 2. This is indicated by the substantially vertical rise of graph 6, and the coefficient of friction represented thereby, and which is due, in part, to lack of uniform impregnation and cure. of the mechanical mixture of sulphur oil employed as the bonding agent.

Fig 3 is a group of graphs which show the results of tests made upon a specimen of a friction element employing woven asbestos tape as friction material and employing as the bonding agent a solution of sulphur in the aforesaid socalled purely polymerized linseed oil, that is to say, heat-polymerized linseed oil from which substantially all of the free fatty acid content formed in the oil during the heat-polymerizing operation was removed therefrom, as formed, thereby providing a partially bodied heat-polymerized linseed oil having a relatively low acid number and substantially free of oxidized oil.

rihe sulphur-oil bonding solution employed was made according to the practice of the present invention and contained 16.7 percent sulphur and 83.3 percent of oil relative to the total weight of the sulphur-oil mixture and after impregnation the woven asbestos-cotton fabric showed a weight increase of 15.6 percent due to impregnation with the sulphur-oil bonding solution.

The preliminary and final curings of the impregnated friction material were made under the same conditions hereinbefore set forth in connection with the description of the graphs shown in Fig. 1 and the specimen of a friction element therein referred to.

As shown in Fig. 3, the specimen of a friction element made in accordance with the practice of the present invention, and which is therein referred to, withstood not only the less severe tests made at application speeds of 1000 feet per minute (graph 7) and 2000 feet per minute (graph 8) but also satisfactorily resisted the more severe test conducted at an application speed of 3000 feet per minute, as shown by the relatively small Vertical rise in graph 9 in Fig. 3 and the correspondingly relatively small increase in coeiicient of friction of the friction element tested. Thus, the superior characteristics of the new friction elements over comparable prior art friction elements may be seen by comparing graph 9 in Fig. 3 with graphs 3 and 6 in Figs. 1 and 2, respectively.

Fig. 4 is a group of graphs also showing the results of tests made upon a specimen of a friction element made in accor-dance with the practice of the present invention. In this instance the sulphur-bonding oil solution employed was the same as that employed in maknig the teststhe results of which are shown in Fig. 3 .and the woven asbestos fabric showed the same percentage -of increase in weight after impregnation. The preliminary and final curings of the bond were conducted under the same conditions as hereinbefore referred to in connection with the description of the graphs shown in Fig, 1.

.conducted at an application speed of 2000 feet per :I

minuta as shown by graph 11 in Fig. 4, but also satisfactorily withstood the severe test conducted :at an application speed of 3000 feet .per minute, as shown by graph 12 in Fig. Li, and which is comparable to and of the same order as graph 9 in .'Fig. 3.

Fig. is a group of graphsshcwing the results of tests made upon a third specimen of a friction element made in accordance with-the practice of `the present invention, and embodying substan tially the same composition as the specimens to which the graphs shown in Figs. 3 and relate and underthe same conditions set forth in connection with the description of the graphs shown in Fig. l ,and the friction element to Which said graphs relate. In this instance, however, the increase in Weight of the woven asbestos tape due to impregnation with the sulphur-oil bonding solution was slightly less than the increase in weight of the friction element specimens to which the graphs `shown in Figs. 3 .and 4 relate, having been 15.1 percent, by weight.

It 'will be seen from the foregoing description, yand from the graphs shown in the accompanying drawings, that the present invention, including the new method of making friction elements, and friction elements made'in accordance therewith, have the desirable characteristics, properties and advantages, and accomplish their intended cb jects, including those Which have been pointed out hereinbefore and others which are inherent in the practive of the present invention.

`While We have described selected embodiments "of .our invention, and a preferred method of rial, and 'a bonding agent comprised of sulphur `and .partially bodied sulphurizable purely heatpolymerized linseed oil, that is, heat-polymerized linseed oil which is substantially free of oxidized oil, and which has been heat-polymerized prior to being mixed with the sulphur until it is highly viscous and tacky but is substantially free from ygel formation and is completely soluble in petroleum thinner, which comprises intimately mixingthe finely divided sulphur and the partially bodied purely heat-polymerized linseed oil in .quantities suflicient to provide a bond for the pre-formed asbestos ber web friction material vand in quantities relative to each other sufficient .toeffect subsequent substantially complete cure of the said `partially bodied purely heat-polymerized linseed oil by sulphurization, dissolving the sulphur in the partially bodied purely heatpolymerized linseed oil by maintaining the said ,sulphur-oil mixture at a temperature of not substantiallyless than 120 C. nor substantially more than :140 C, for a period of time sufficient to completely ldissolve the sulphur in the said partial'lylbodied purely heat-polymerized linseed oil but insuiiicient tocause any substantial reaction between the sulphur yand the said partially lbodied purely heat-.polymerized linseed oil, impregnating the said pre-formed asbestos fiber web friction material with the thus formed sulphur-oil bonding solution, and then heating the thus impregnated preformed asbestos ber web friction material at a .temperature and for a period of time sunicient to completely solidify the bond by sulphurization in situ. k

y2. A'bonding agent for friction elements which is comprised of a solution formed byintimately mixing finely divided sulphur and partially bodied purely heat-polymerized linseed oil, that is, heatpolymerized linseed oil which is substantially. free of oxidized oil and which has been heat-polymerized, prior to being mixed with the sulphur, until itis highly viscous and tacky but is substantially free of gel formation and is compl tely solua'ble in petroleum thinner, and heatin the sulphuroil mixture thus formed ata temp ratureof not substantially less than Q C. nortsubstantially more than C., for a period of Vtime suicient to completely dissolve the sulphur in the yoil but insufficient to cause any substantial reaction therewith.

3. The method of forming a bonding agent for friction elements which comprises intimately mixing finely divided sulphur and partially bodied purely heat-polymerized linseed oil, that is. heat- `po'lymerized linseed oil which issubstantially free of oxidized oil and Which has beenv heat-polymerized, prior to being mixed with the sulphur, until it is vhighly viscous and tacky but is substantially free from gel formation and is completely soluble in petroleum thinner, in quantities suiicient'relative to each other to provide a bonding agentfor friction elements, and then dis'- solving the said sulphur in the said partially bodied purely heat-polymerized linseed oil by maintaining the said sulphur-oil mixture -at a temperature of not substantially less than 120g` C. or lsubstantially more' than 140 C. for a period of time sucient to completely dissolve the sulphur in the said partially "bodied purely heatepolymerized linseed oil but insuflicient to cause any substantial reaction between the sulphur and the said partially bodied purely heat-polymerizedlin*- oil.

WLLIAM A. BLUME. RAY E. SPOKES.

REFERENCES @FEED 'The following references are of record in the nle of this patent:

UNITED STATES PATENTS Great Britain 1 931 

